1. Field of the Invention
The present invention relates to a bump formation method for use in the mounting of semiconductor elements, in the connection of wires, etc. and to a wire bonding method for performing wire bonding between two conductors.
2. Prior Art
The method shown in FIG. 5 (see Japanese Patent Application Publication (Kokoku) No. 6-95468 (corresponding to Japanese Patent Application Laid-Open (Kokai) No. 63-304587)) and the method shown in FIG. 6 (see Japanese Patent No. 2735022 (corresponding to Japanese Patent Application Laid-Open (Kokai) No. 8-264540)) are the conventional bump formation methods that allow the formation of bumps with a large height in an extremely simple manner.
In the bump formation method shown in FIG. 5, as seen from the illustration of step (e) in FIG. 5, the bottom portion 20 of a projection-form contact point is formed on top of an electrode pad 2 on the surface of a semiconductor element 1, and the top portion 21 of a ring-form or inverted U shaped projection-form contact point formed as a continuation of this bottom portion 20 on top of the bottom portion 20.
The formation process is as follows: first, in step (a) in FIG. 5, a ball 22 is formed by means of a discharge spark, etc. on the tip end of a wire 4 that passes through a capillary 1. Next, in step (b), the capillary 3 is lowered, and the ball 22 is joined to the surface of the electrode pad 2 by hot press-bonding or ultrasonic vibration, etc., so that the bottom portion 20 of the projection-form contact point is formed. Next, in step (c), the capillary 3 is moved in a loop form, and then, in step (d), the capillary 3 is lowered, the wire 4 is connected to the upper surface of the bottom portion 20, and the wire 4 is cut, so that the top portion 21 which has a ring-form shape or inverted U shape is formed.
In the bump formation method shown in FIG. 6, as seen from the illustration of step (i), a first bump 30 is formed on top of an electrode pad 2 on the surface of a semiconductor element 1, and a second bump 31 is formed on top of this first bump 30, so that a two-stage bump 32 is formed.
The formation process in this case is as follows: first, in step (a), a ball 33 is formed by means of a discharge spark, etc. on the tip end of a wire 4 that passes through a capillary 3. Next, in step (b), the capillary 3 is lowered, and the ball 33 is joined to the surface of the electrode pad 2 by hot press-bonding or ultrasonic vibration, so that a first bump 30 is formed. Next, in step (c), the capillary 3 is raised, and the capillary 3 is moved in the horizontal direction so that a flat portion on the tip end of the capillary 3 is positioned so as to face the center of the first bump 30.
Next, in step (d), the capillary 3 is lowered and the first bump 30 is formed; then, a cut-out 34 is formed in the wire 4 by the application of ultrasonic waves, after which the capillary 3 is raised and the wire 4 is cut, thus forming the first bump 30. Then, in step (e), a ball 35 is formed on the tip end of the wire 4. Subsequently, a second bump 31 is formed on top of the first bump 30 by the steps (f) through 6 (h), in the same manner as in the steps (b) through 6 (d) shown in FIG. 6.
In the method of Japanese Patent Application Publication (Kokoku) No. 6-95468 shown in FIG. 5, the shape of the top portion 21 is in a ring-form shape or inverted U shape. However, since there is a clearance between the internal diameter of the hole in the tip end of the capillary 3 and the external diameter of the wire, slack is generated in the wire 4 during the driving of the capillary 3, so that the top portion 21 cannot be obtained with a uniform ring-form shape of inverted U shape. Moreover, there is a variation in the orientation of this shape.
In the method of Japanese Patent No. 2,735,022 shown in FIG. 6, in cases where a two-stage bump 32 (including bumps 30 and 31) is formed, two steps that form the balls 22 and 33 and two steps that cut the wire 4 are required. In other words, since a number of ball formation steps and wire cutting steps corresponding to the number of stages in the bump are required, this method has a problem of poor productivity.